1. Field of the Invention
The present invention relates to a semiconductor-encapsulating resin composition and a semiconductor device.
2. Discussion of the Background
In the recent market trend of decreasing size, decreasing weight, and increasing performance of electronic devices, the degree of integration of semiconductor elements (hereinafter also referred to as “elements” or “chips”) is increasing year by year, and surface mounting of semiconductor devices (hereinafter also referred to as “packages”) has also been accelerated. Accordingly, the requirements for a semiconductor-encapsulating resin composition (hereinafter also referred to as “encapsulating material” or “molding compound for encapsulating”) have become more severe. In particular, in a current typical surface mounting process of a semiconductor device, when a semiconductor device that has absorbed moisture is exposed to a high-temperature atmosphere during a soldering process, the semiconductor device receives explosive stress due to vaporized steam. As a result, cracks are formed in the semiconductor device, or detachment occurs at the interface between the semiconductor device or a lead frame and a cured product of a semiconductor-encapsulating resin composition. In such a case, the electrical reliability of the final semiconductor device may be significantly impaired. Accordingly, prevention of such defects, more specifically, an improvement in solder resistance of a semiconductor-encapsulating resin composition has been strongly desired. Furthermore, with the continuing elimination of use of lead, the frequency of use of lead-free solder, which has a melting point higher than conventional solder, has been increasing. In the case where such lead-free solder is used, it is necessary to increase the temperature during semiconductor mounting by about 20° C. compared with the case where conventional solder is used. When a semiconductor is mounted at such a high temperature, the reliability of the semiconductor device after mounting is significantly degraded, as compared with a conventional case. In addition, in view of environmental problems, a desire for imparting flame resistance to a encapsulating resin without using a flame retardant such as a Br compound or antimony oxide has also been increasing.
In order to improve solder resistance and flame resistance, it is believed that the use of a encapsulating resin containing a large amount of an inorganic filler is effective. The inorganic filler decreases the water-absorbing property of a resin and improves solder resistance. Furthermore, by using an inorganic filler in a large amount, the content of a flammable resin component in the resin can be decreased to improve flame resistance. In view of this circumstance, recent semiconductor-encapsulating resin compositions contain a crystalline epoxy resin having a lower viscosity and a larger amount of inorganic filler so as to have a high flowability with a low viscosity during molding. For example, Japanese Unexamined Patent Application Publication No. 7-130919 and Japanese Unexamined Patent Application Publication No. 8-20673 disclose such semiconductor-encapsulating resin compositions. The contents of these publications are incorporated herein by reference in their entirety. However, a method that can combine satisfactory solder resistance with satisfactory flame resistance during mounting has not yet been known.
Consequently, the present applicant has proposed a biphenylene-structure-containing phenol-aralkyl-type epoxy resin and an epoxy resin composition containing a biphenylene-structure-containing phenol-aralkyl-type curing agent that have excellent solder resistance and flame resistance. For example, Japanese Unexamined Patent Application Publication No. 11-140277 discloses such epoxy resin or epoxy resin composition. The contents of this publication are incorporated herein by reference in their entirety. This epoxy resin composition contains a large number of aromatic rings in its molecular structure, and thus, a carbonized layer is formed on the surface layer of a molded product of the resin composition during burning. Accordingly, further burning can be suppressed, and the resin composition exhibits excellent flame resistance. Furthermore, the presence of the aromatic rings improves hydrophobicity of the composition. Furthermore, the presence of the aromatic rings increases the distance between cross-linking points of the resin. As a result, the modulus of elasticity of the composition at high temperatures is decreased, thereby improving solder resistance.
Recently, further improvements in solder resistance and flame resistance of a encapsulating resin have been desired. It is believed that a solution to realize this desire is to increase the content of an inorganic filler.
In addition, with a recent increase in the number of I/O terminals of semiconductor devices, the pitch of bonding wires that electrically connect a silicon chip to a circuit on a substrate tends to be decreased. Therefore, high flowability of encapsulating resins is desired. However, the melt viscosities of biphenylene-structure-containing phenol-aralkyl-type epoxy resins and biphenylene-structure-containing phenol-aralkyl-type curing agents which are used in the encapsulating resins are not low, and thus, the flowability of the encapsulating resins is decreased. Consequently, a method in which the melt viscosity is decreased by decreasing the molecular weight of the resins is often employed. However, this method often impairs the curability of the resins.